Facebook Dilemma...

I love connecting with friends, but sometimes Facebook can also be a curse. I've often struggled at whether or not to accept friend requests from someone I don't remember, but they are listed as going to my high school, or someone I know but barely, etc. However, today I got a Facebook request that has left me with absolutely no idea what to do. I remember this person, perhaps a little too well. It is from the only mission companion I had who I am convinced absolutely hated me. Let me call her Sister X. I half expected Sister X to murder me in my sleep. She was meaner to me than anyone has been since Greg C. harassed me in front of all the boys in the 6th grade. I am not saying I was perfect, but we talked as a companionship about what was going wrong, and I tried my absolute hardest to work on all the problems she had with me, and all my efforts were flatly rejected. We were actually in a threesome at the time (I know, now you think it was being in a threesome that was the problem; that's probably partly true, but I was in 11 different threesome's on my mission and several of them were very good companionships). The other sister (Sister Y) somehow got along just fine with Sister X. However, even she told me in confidence once that Sister X wasn't being fair to me. Looking back I realize that the problem was Sister X was emotionally abusive. Trust me, I wouldn't say that lightly; I thought about it a lot. Anyway, I think she felt threatened by me for some reason and her lack of self-confidence lead her to constantly rag on me. I was always doing stuff wrong, and ruining everything, and I was pretty much completely worthless according to Sister X. She liked to remind me of these things often; at least when she wasn't too busy ignoring me.

So back to my initial problem; today I received a friend request and a message on Facebook, "Hi Sister Sabin, this is Sister X!" I'm just not exactly sure what to do with it. I don't exactly hold a grudge against her; I've just spent a lot of time forgetting her all together. I liked it that way. For the past six years since I've been home I have been quite happy to think of her almost never. Now that she seems for some odd reason to want to be my Facebook friend, I have no idea what to do. Do I accept her friend request to be nice, and then just try to forget her again? Do I simply ignore the request and move on? Do I send her a message saying, "why in the world do you want to be my Facebook friend anyway?" It's crazy. Like she has absolutely no idea how incredibly mean she was to me. Actually, that's probably it. She probably doesn't remember at all. However, with someone that manipulative, I wouldn't put something more conniving past her. Maybe she just wants to be able to rub something in...
What do I do? Advice please!